This invention relates to a crucible for growing large macrocrystals (or ingots) from a melt in a highly modified Stockbarger type furnace, or a comparable one in which slow growth of a solidifyng macrocrystal from the melt under the influence of the earth's gravity, results in the solidified macrocrystal being supported within the walls of the crucible. It is accepted practice to refer to melt-grown isometric blanks as being "single" when the finished crystal does not exhibit flaws attributable to multiple components disregarding mosaics spread which is never less than 0.250.degree. in the flattest cleavage and can generally be seen in the texture of thermal etch. In fact a mosaic spread of 3.degree. is valued in pulled crystals for not showing the following flaws attributable to multiple components though their displacement may be less than 1.degree.. However the displacement may be any angle up to 90.degree. and still be optically homogeneous when not decorated by inclusions.
Multiple components are said to be present when they are visible as a fringe pattern, or strain pattern in an optically polished crystal. Multiplicities will fracture non-cubic crystals while mosaics will not. Mosaic spread is important for X-ray plates and gun sights. Mosaics which do not have unacceptable flaws are acceptable in a single crystal.
An ideal macrocrystal is a single crystal, but typically a macrocrystal grown is only optically monocrystalline, being formed of several crystals demarcated by boundaries within the macrocrystal. Grain boundaries within a large macrocrystal (an ingot) which are discontinuities are undesirable.
More specifically the novel crucible is useful to grow crystals of halides of an element of Group 1a and Group 2a of the Periodic Table, particularly the alkali metal fluorides and alkaline earth metal fluorides, lead fluoride and crystals of the foregoing salts which are doped with desirable metal ion dopands. High quality macrocrystals of fluorides of lithium and sodium, as well as of magnesium, calcium, barium and strontium have been produced commercially since soon after 1939 when Donald Stockbarger disclosed a method for their manufacture in U.S. Pat. No. 2,149,076 and we taught methods for purifying melts in U.S. Pat. Nos. 2,498,186 and 2,550,173. However, producing high quality macrocrystals depends upon many factors, including having the skills required to duplicate successful runs. Success depends upon anticipating the need for minute adjustments in "power hours" (rate at which power is delivered to heat the melt), among others, all of which adjustments must be made before a probe inserted -in the melt shows any indication of a change in the rate of growth.
To date, a graphite crucible may be used for growing a Stockbarger macro-crystal provided the graphite is not so porous as to allow the melt to leak through it. A crucible may have an inclined or flat bottom and either might be provided with a well for a "seed crystal" holder.
U.S. Pat. No. 5,911,824 teaches that a particular graphite crucible could not contain a melt of thallium iodide doped sodium iodide, NaI(Tl) (see col 4, lines 26-34). A Nal(TI) crystal was successfully grown in, a graphite crucible the inside surface of which is coated with graphitic pyrolitic carbon, and the crystal did not adhere to the coated surface (see col 4, lines 36-47).
However, a second crystal grown. in the same crucible, adhered strongly to the crucible and could not be removed in its entirety without a remelting procedure (see col 4, lines 60-66). If the crystal was grown in a platinum crucible, the crystal adheres to the platinum. The platinum crucible must be heated to melt the surfaces of the crystal in contact with the platinum (referred to as "remelting") before the platinum crucible may be slid off the crystal.
It is a characteristic of macrocrystals of the aforementioned fluorides of melt-grown metal or metal-like elements, that they shrink when they solidify (referred to herein as "shrinking melts"), and the first crystal grown may be readily removed because the walls of a graphite or carbon crucible are not wetted by the melt.
To be readily removed, sufficient shrinkage must occur when, and after, the melt solidifies provided also that the bond between crystal and crucible is weaker than the force needed to fracture the crystal as it cools. Since the coefficient of linear expansion of graphite is about 7.35 microinclies/(inch)(.degree.C.) measured at about 40.degree. C., that of the salt must be greater, preferably at least 5% greater, and more preferably 10% greater. The coefficient of fluorspar is about 19.5 micro-inches/(inch)(.degree.C.) measured at about 40.degree. C., so that even in a relatively small crucible having an inside diameter in the range from about 5 cm to about 25 cm, the circumferential surface of the solidified crystal pulls away from the inner walls of the crucible, and the crystal may be lifted out of the crucible with a vacuum cup without breaking the crystal or crucible., even when the crucible is cylindrical and its sides are vertical. This may be done provided the surface tension of the melt is high enough so as to fail to substantially wet the walls of the crucible; because the melt does not wet the surface of the graphite it does not to seep into its pores. Note that the linear coefficients of expansion at melt temperature will likely be substantially different from those given above.
The effect of such shrinkage on a crystal no larger than about 7.5 cm at its greatest diameter (referred to as a 7.5 cm diameter macrocrystal) is not particularly notable even if the shrinkage is not controlled, but for larger crystals that effect is; the larger the crystal, the more detrimental are the effects of such uncontrolled shrinkage.
Using a shrinking melt to grow a Stockbarger macrocrystal typically comprises slowly moving the melt at a controlled rate from a region hotter than its solidification temperature to a region cooler than its solidification temperature, controlling the relative temperatures of the regions, and maintaining a temperature gradient in a localized zone between the regions at the boundary of the melt. The temperature gradient in the zone is sufficient to allow melt to crystallize at the cooler boundary of the localized zone. An "elevator" type furnace may be used where the crucible is raised or lowered on an elevator; or a "movable temperature gradient" furnace may be used where the furnace is moved and the crucible is stationary. The gradient between melt near the top of a crystal and the sharply localized zone is in the range from about 100.degree. C. to about 500.degree. C. depending upon the particular halide.
It will be evident that the temperature of the edge portion of successive layers of the melt corresponds to the solidification temperature of the melt as these edge portions reach a substantially fixed location in the path of travel of the melt and solidification begins and progresses inwardly. Preferred crystals are obtained when the zone of solidification approximates a plane. It is desirable to control the rate of heat flow through the inner portion of the melt from the hotter to the cooler region. If the rate of heat flow through the inner portion of the melt is too slow, the zone of solidification tends to be concave. Properly controlling the rate of heat flow through the inner portion of the melt allows the zone of solidification to approximate a plane.
To date, the art has addressed the problem of a macrocrystal adhering to the inner surface of the crucible by either providing a very smooth microporous graphite surface, or by coating the surface of the graphite as in the '824 patent, or by lining a mechanically stable temperature resistant material such as alundum or graphite with a thin sheet of platinum as in U.S. Pat. No. 5,997,640. In either case, the better is the separation upon solidification, the more readily the macrocrystal falls to the bottom of the crucible. In a crucible in which the porosity is such that the melt seeps partially into the pores for a short distance sufficient to hold the weight of the crystal, the bottom of the solidified crystal pulls away from the bottom of the crucible, leaving a bottom gap. In many instances the seepage is insufficient to support the weight of the crystal in the walls of the crucible and the crystal will suddenly drop to the bottom of the crucible closing the gap. Such partial seepage into the pores is deemed to fail substantially to wet the interior surface of the crucible.
Making an uncontrolled change in the flow of heat and the rate of growth of the crystal within the crucible gives rise: to "sparkles". Sparkles are especially evident in ingots larger than 7.5 cm effective diameter, this being the longest diameter in a horizontal plane. With effective diameters smaller than about 7.5 cm, either the ingot does not drop or the flaws are lost with others in the "heel" of the ingot. Small ingots generally lack sparkles because minute, micron-size gas inclusions must migrate upwards to gain in size sufficient to develop reflecting facets and become visible. The "heel" refers to the base oiC an ingot which has been removed from the crucible in which the ingot was grown, and placed upside-down, so that the ingot rests on its upper surface; while the ingot was being formed the heel was at the top of the ingot. The upper portion refers to a zone above the mid-horizontal plane through the macrocrystal; the upper portion terminates in a heel demarcatable from the upper portion. The upper portion of an ingot grown in the novel crucible is essentially free from gas-induced sparkles.
In many instances the number of multiplicities is not of great concern, but in those instances where an essentially single crystal is desired, it is generally hoped that number is minimized by seeding with a single crystal. The novel crucible of this invention provides a far more reliable means for minimizing the number of multiplicities in an ingot, particularly relatively large ones which to date, are never single.
Relatively large ingots, particularly those larger than about 15 cm (6 ins.) in effective diameter, are highly susceptible to flaws due to gas inclusions, caused by small pockets of gases trapped within the ingot as it is solidifying. Such inclusions are referred to in the art as "sparkles" because these inclusions reflect flashes of light, or glitter, within the crystal when light is shone through the crystal. It is well established that sparkles are voids bounded by flat 111 facets because polarized light shows no strain that even the smallest solid would generate from different contraction rates. In CaF.sub.2 grown in a Stockbarger furnace, sparkles are evidenced by flat six-sided facets. Migration of the voids (upward in a Stockbarger ingot) toward the hotter zone allows these voids to combine and spread, forming a broad generally horizontal band. In relatively small barium fluoride (BaF.sub.2) ingots 15 cm (6 ins) in diameter some sparkles grow to 5 mm in -width while others remain less than 1 .mu.m wide, all reflecting from 111 facets set by the host crystal.
In an essentially perfect crystal lattice there is no tolerable physical location in the lattice of ordered atoms for gas molecules. Any interruption which causes faster growth will trap dissolved gases on a molecular scale. While the crystal is cooling, and within about 50.degree. C. of its melting point, trapped molecules of gases migrate toward the hotter zone. Such molecules of trapped gases are mainly carbon monoxide (CO) and hydrogen (H.sub.2). Growing an ingot of an alkali metal or alkaline earth metal fluoride in a graphite crucible may provide, in addition, carbon tetrafluoride (CF.sub.4) and other fluorocarbons probably by the action of PbF.sub.2 (used as a getter) vapor on the very hot graphite heater along with molecules of nitrogen (N.sub.2) from the air. Leakage of air is held to a minimum to avoid oxygen-stabilized color centers in ingots of alkaline earth fluorides, and others. Carbon monoxide (CO) and CF.sub.4 should be more soluble in these melts than hydrogen (H.sub.2).
In a platinum crucible, the ingot shrinks after it solidifies but still adheres to the inner surface of the platinum crucible, contracting not only the sides but also the bottom of the crucible. This is the reason why the platinum crucible is heated sufficiently to free the ingot so that its contraction will not crack it. Just the contraction during growth shrinks the platinum crucible so that it must be rolled on a mandrel to stretch it to its initial size before it is reused. I concluded that the more uniform and better heat transfer through the walls and bottom of the platinum crucible was the reason why an ingot grown in a platinum crucible was substantially free from "sparkles" caused by gas inclusions. Also, adhesion of the solidifying ingot to the platinum prevents any movement inside the ingot during its growth.
It should be noted that an ingot grown in a platinum crucible may have other, non-gas derived inclusions, also referred to as "sparkles", generated by the intense radiation of the "remelt furnace" (used specifically to remelt the adhering surface of the ingot) when, after the platinum crucible is removed, the ingot continues to be heated. Such sparkles are the result of the recrystallization of a small volume of melt generated within the solidified ingot by radiant heat. Such "non-gas derived" sparkles are visually distinguished from those caused by gas inclusions; non-gas derived sparkles appear as 8-pointed "stars", the points radiating in the 1,1,1 directions, and have no flat facets. This is attributable to their having formed so fast they did not have time to grow facets.
Another kind of "sparkles" commonly found near the bottom of an ingot are platinum sparkles which are very small, about 1-5 .mu.m, formed as a metallic moss. Their location alone sets them apart from gas-induced, or typical non-gas derived sparkles. Such platinum sparkles would not be expected in ingots grown in graphite crucibles but for the fact that platinum is used in the preparation of growth stock.
Since solidification of even a relatively small ingot is very slow, the bottom of the charge in the crucible becomes solid, giving form to the ingot while the upper portion of the charge is still molten and tightly held within the upper portion of the walls of the crucible because the upper portion is not cool enough to shrink. The growing ingot, upon cooling sufficiently, contracts and becomes spaced apart from the graphite floor. When the upper portion is finally cool enough to shrink, the ingot falls and rests on the floor of the graphite crucible. The timing of this "ingot drop" is unpredictable. The result is that in the initial portion of growth of the ingot, its bottom is in contact with the graphite bottom of the crucible affording excellent and predictable heat transfer; in the intermediate portion of the growth, the bottom of the ingot is spaced apart from the graphite floor of the crucible resulting in poor but predictable heat transfer; and in the last portion of the growth the bottom of the ingot is again in contact with the floor of the graphite crucible affording excellent heat transfer. The unpredictability of when the heat flow changed results in poor control of growth and inclusion of gas.
As indicated above, transversely spaced apart indentation means adapted to support an ingot near its edges and maintain its bottom in spaced apart relationship with the floor of the crucible, is an effective means to control the temperature gradient through the solidifying ingot and such control is found to minimize sparkles. Another means for obtaining the controllable temperature gradient is to configure a graphite crucible so that an ingot grown from a shrinking melt is continuously in contact with the floor of the crucible, instead of spaced apart from it. Such configuration provides an equally effective solution to the problem of sparkles due to gas inclusions in an ingot. In either case, controlled solidification is maintained slowly enough to allow gas molecules to diffuse out of the lattice being formed, and upwards out of the melt.